In known types of nuclear reactors, such as boiling water reactors (BWR), the reactor core comprises a plurality of fuel assemblies arranged in an array capable of self-sustained nuclear fission reaction. The core is contained in a pressure vessel and submerged in water, which serves as both a coolant and a neutron moderator. A plurality of control rods containing neutron absorbing material are insertable in gaps between the fuel assemblies to control the reactivity of the core. Each fuel assembly includes a flow channel through which water is pumped upwardly from a lower plenum to an upper plenum. To monitor the power density of the core, it is common practice to distribute neutron detectors both radially and axially throughout the core. The signals from these neutron detectors are utilized to monitor core conditions and to initiate corrective actions, including reactor shutdown (SCRAM), in the event of a detected abnormality.
One reactor abnormality that has come under close scrutiny due to recent events is thermal-hydraulic instability. As water is pumped upwardly through the fuel assembly flow channels, vaporization occurs. The resulting vapor bubbles are in constant motion, ever expanding and contracting. This produces variations in the two-phase fluid flow through the channels. If these flow variations are not dampened or suppressed by normal flow losses due to friction, they can build into sustained oscillations. Since the fluid is also a neutron moderator, flow oscillations will result in neutron flux oscillations and thus power oscillations along the vertical length of the fuel assemblies. With recent changes in fuel neutronic and heat transfer characteristics, such thermal-hydraulic induced power oscillations could conceivably exceed minimum critical power ratio (MCPR) safety limits.
Such neutron flux oscillations, which have been determined to only occur under high power and low coolant flow operating conditions, are of basically two modes. One mode is a core wide oscillation, wherein all fuel assemblies participate in phase with each other in the oscillation. The second mode is a regional oscillation, wherein the neutron flux on one side of the core oscillates out-of-phase with the neutron flux on the other side. The axis of zero oscillation magnitude may be at any angle relative to the X-Y (horizontal) fuel bundle plane, rotate in the X-Y plane, or the two regions of the core of peak oscillation amplitude may shift from one location to another at a frequency slower than the oscillation frequency.
Existing in-core power monitoring instrumentation has been largely directed to monitoring average power by averaging the signals from selected neutron detectors widely distributed within the core. While such average power range monitoring (APRM) systems can detect and initiate action to suppress unacceptably high core-wide neutron flux oscillations, they do not reliably detect regional oscillations, since averaging detector signals that are relatively out of phase results in substantial cancellation.
There is thus an important need for an oscillation power range monitoring (OPRM) system that can detect the onset of both core-wide and regional neutron flux oscillations and reliably initiate an automatic suppression function (ASF) to suppress an oscillation prior to its exceeding safety limits. Such an OPRM system must also distinguish between instability related oscillations and oscillations resulting from normal reactor events, such as control rod maneuvers and pressure regulator transients, to avoid unnecessary ASF initiation. Also sufficient redundancy must be built into the system to accommodate a certain number of inoperative neutron detectors and/or spurious detector signals and still reliably detect instability-related neutron flux oscillations.